1. Field of the Invention
The present invention relates to a belt driving apparatus, fixing apparatus and image forming apparatus.
2. Description of the Related Art
Among the conventional electrophotographic image forming apparatuses are printers, copying machines, and facsimile machines. For printers, a charging unit charges the entire surface of a photoconductive drum to a uniform potential. Then, an exposing unit illuminates the charged surface to form an electrostatic latent image. A developing unit develops the electrostatic latent image into a toner image. The toner image is then transferred onto a recording paper. The recording paper having the toner image thereon is then advanced to a fixing unit where the toner image is fixed into a permanent image.
The fixing unit includes a fixing roller and a pressure roller in pressure contact with the fixing roller. When the recording paper is pulled in between the fixing roller and the pressure roller, the fixing roller heats the toner image and the pressure roller presses the toner image against the recording paper. In order to increase printing speed, the amount of heat supplied to the toner image needs to be increased.
For this purpose, a belt type fixing unit has been proposed which includes a heat roller in addition to a fixing roller and a pressure roller. An endless belt is sandwiched between the fixing roller and heater roller unit. A relatively large nip is formed between the belt and pressure roller, and the heat roller and pressure roller are heated to heat the belt.
If the belt runs crooked due to dimension errors in various members, imbalance of tension applied to the belt, and non-uniform temperature distribution across the length of the fixing roller and heat roller, the edge portion of the belt will be damaged. In order to solve this problem, a flange is provided at the longitudinal ends of the heat roller and serves as a stopper that prevents the belt from shifting toward one ends of the fixing roller and heat roller.
FIG. 11 illustrates a conventional belt. FIG. 12 and FIG. 13 illustrate how the belt shifts to one side.
Referring to FIGS. 11-13, a flange 13 is disposed on one end of a heat roller 11. The heat roller 11 has flanges 13 at its longitudinal ends and the flanges 13 are rotatably supported by bearings 14. The bearing 14 is supported by a supporting frame 15. The heat roller 11 rotates in a direction shown by arrow A. A belt 12 is entrained about the heat roller 11 and runs in a direction shown by arrow B.
Referring to FIG. 12, when the belt 12 shifts toward one longitudinal end of the heat roller 11 to touch the flange 13, a frictional force is exerted on the belt 12 to cause the edge portion of the belt 12 to deform in a direction shown by arrow C at a point P1 where the belt 12 contacts the flange 13. Prolonged application of such a frictional force will eventually cause the edge portion of the belt 12 deform greatly in a radial direction of the flange as shown in FIG. 13, so that the belt runs over the flange 13 or becomes damaged.